Magnetic domain fluctuations in an antiferromagnetic film observed with coherent resonant soft x-ray scattering

We report the direct observation of slow fluctuations of helical antiferromagnetic domains in an ultra-thin holmium film using coherent resonant magnetic x-ray scattering. We observe a gradual increase of the fluctuations in the speckle pattern with increasing temperature, while at the same time a static contribution to the speckle pattern remains. This finding indicates that domain-wall fluctuations occur over a large range of time scales. We ascribe this non-ergodic behavior to the strong dependence of the fluctuation rate on the local thickness of the film.Comment: to appear in Phys. Rev. Let

Direct observation of t2g orbital ordering in magnetite

Using soft-x-ray diffraction at the site-specific resonances in the Fe L23 edge, we find clear evidence for orbital and charge ordering in magnetite below the Verwey transition. The spectra show directly that the (001/2) diffraction peak (in cubic notation) is caused by t2g orbital ordering at octahedral Fe2+ sites and the (001) by a spatial modulation of the t2g occupation.Comment: to appear in Phys. Rev. Let

Spectroscopy of stripe order in La1.8Sr0.2NiO4 using resonant soft x-ray diffraction

Strong resonant enhancements of the charge-order and spin-order superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when x-ray energies in the vicinity of the Ni L2,3 absorption edges are used. The pronounced photon-energy and polarization dependences of these diffraction intensities allow for a critical determination of the local symmetry of the ordered spin and charge carriers. We found that not only the antiferromagnetic order but also the charge-order superstructure resides within the NiO2 layers; the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the cuprates.Comment: 4 pages, 3 figure

Epitaxial and layer-by-layer growth of EuO thin films on yttria-stabilized cubic zirconia (001) using MBE distillation

We have succeeded in growing epitaxial and highly stoichiometric films of EuO on yttria-stabilized cubic zirconia (YSZ) (001). The use of the Eu-distillation process during the molecular beam epitaxy assisted growth enables the consistent achievement of stoichiometry. We have also succeeded in growing the films in a layer-by-layer fashion by fine tuning the Eu vs. oxygen deposition rates. The initial stages of growth involve the limited supply of oxygen from the YSZ substrate, but the EuO stoichiometry can still be well maintained. The films grown were sufficiently smooth so that the capping with a thin layer of aluminum was leak tight and enabled ex situ experiments free from trivalent Eu species. The findings were used to obtain recipes for better epitaxial growth of EuO on MgO (001).Comment: 10 pages, 15 figure

Epitaxy, stoichiometry, and magnetic properties of Gd-doped EuO films on YSZ (001)

We have succeeded in preparing high-quality Gd-doped single-crystalline EuO films. Using Eu-distillation-assisted molecular beam epitaxy and a systematic variation in the Gd and oxygen deposition rates, we have been able to observe sustained layer-by-layer epitaxial growth on yttria-stabilized cubic zirconia (001). The presence of Gd helps to stabilize the layer-by-layer growth mode. We used soft x-ray absorption spectroscopy at the Eu and Gd M4,5 edges to confirm the absence of Eu3+ contaminants and to determine the actual Gd concentration. The distillation process ensures the absence of oxygen vacancies in the films. From magnetization measurements we found the Curie temperature to increase smoothly as a function of doping from 70 K up to a maximum of 125 K. A threshold behavior was not observed for concentrations as low as 0.2%.Comment: 8 pages, 9 figure

Analysis of charge and orbital order in Fe_{3}O_{4} by Fe L_{2,3} resonant x-ray diffraction

To elucidate charge and orbital order below the Verwey transition temperature TV∼125 K, a thin layer of magnetite partially detwined by growth on the stepped MgO(001) substrate has been studied by means of soft x-ray diffraction at the Fe L2,3 resonance. The azimuth angle, incident photon polarization, and energy dependence of the (0012)c and (001)c reflection intensities have been measured, and analyzed using a configuration-interaction FeO6 cluster model. The azimuth dependence of the (0012)c reflection intensities directly represents the space-group symmetry of the orbital order in the initial state rather than indirectly through the intermediate-state level shifts caused by the order-induced lattice distortions. From the analysis of the (0012)c reflection intensities, the orbital order in the t2g orbitals of B sites below TV is proved to have a large monoclinic deformation with the value of Re[Fxy]/Re[Fyz]∼2. This finding contradicts the majority of theories on the Verwey transition so far proposed. We show that the experimentally observed resonance spectra cannot be explained by orbital and charge orders obtained with recent LDA+U and GGA+U band structure calculations but by a complex- number orbital order with excellent agreement